Molecular cloning of a genetic determinant for Congo red binding ability which is essential for the virulence of...

A DNA sequence of about 1.0 kilobase (kb) derived from a 230-kb (140-megadalton) plasmid in a fully virulent Shigella flexneri 2a strain, YSH6000, was cloned into Escherichia coli K-12 by selecting for the ability to bind Congo red (Pcr+ phenotype). It was mapped and localized within the SalI restriction fragment F on the plasmid. This clone converted an S. flexneri strain which is avirulent as a result of a small deletion in the plasmid to full virulence but did not convert those without the 230-kb plasmid or with a plasmid bearing a more extensive deletion. This indicates that there are more than two virulence determinants on the plasmid. Thus, this sequence contains a genetic determinant which is essential but not sufficient for full virulence and expression of the Pcr+ phenotype in S. flexneri but is essential and sufficient for expression of the Pcr+ phenotype in E. coli K-12. We noted that there exist some other regions on the 230-kb plasmid which express the Pcr+ phenotype in E. coli K-12. Although these regions express the Pcr+ phenotype less markedly than the region cloned in the present study, they do hybridize with it.